The aim of joint cartilage repair is to restore the integrity of the joint surface, reduce pain and prevent any further deterioration of the tissues.
Joint cartilage is a tissue which allows virtually frictionless movement of the joint. Its particular biological characteristics enable the joint to absorb forces at least five times greater than the body's weight. The joint cartilage, or hyaline, has a very limited capacity for self-repair, so the type of cartilage that is spontaneously regenerated after damage does not possess the same characteristics as the original tissue. It is known as fibrocartilage and has no properties of lubrication or absorption of mechanical shock. The final phase of hyaline cartilage degeneration is accompanied by pain and limited mobility that may cause locking of the joint. In the long term, the degenerative process may even cause the onset of complications such as osteoarthritis. In the most severe cases, the joint, usually the knee, has to be replaced with a metal prosthesis. This is a costly procedure and is not even permanent because many prostheses have to be changed after about 10-15 years. For this reason, knee replacements are only performed as a last resort in patients of under 50 years old. Joint cartilage lesions are currently treated by arthroscopic surgical techniques chiefly aimed at reducing pain, slowing down the degeneration process and, whenever possible, repairing the damage. Many methods have been applied to date to treat cartilage defects, and each of them has certain disadvantages (T. Minas et al. “Current concepts in the treatment of articular cartilage defects”, Orthopedics, June 1997, Vol. 20 No. 6). One such technique involves trimming the margins of the cartilage defect, in other words, débridement of the edges of the lesion by removing any necrotic or diseased tissue. The technique of stimulating the marrow consists in reaching areas of the subchondral bone tissue by abrasion or perforation, thus stimulating the formation of a fibrin clot containing pluripotent stem cells. The clot then differentiates and takes form, giving rise to fibrocartilage repair tissue. The resulting tissue does not, however, have the mechanical properties or physiological or structural characteristics of healthy, lasting joint cartilage.
Another technique consists in implanting a piece of periosteum or perichondrium tissue, taken for example from rib cartilage, into the defect. Initially, this treatment triggers the development of hyaline cartilage, but the repair tissue does not take easily to the surrounding healthy tissues, and subsequently becomes ossified. Recently, a team of Swedish researchers devised an ex-vivo technique of grafting autologous chondrocytes, where chondrogenic cells are isolated from a small cartilage biopsy, grown in vitro and then regrafted in the same subject (M. Brittberg, A. Lindahl, A. Nilsson: “Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation”, N. Eng. J. Med: 1994, 331, 889-895). According to the authors, in the culture phase, the chondrocytes temporarily de-differentiate and multiply under stimulation by suitable growth factors. Once transplanted to the damaged area, they recover their phenotype memory and consequently re-differentiate into chondrocytes able to produce a hyaline-type cartilage matrix. The surgical procedure is actually rather complex. First of all, the operation requires open surgery. Moreover, the cartilage defect must be well located and covered by a lid of periosteum (taken in the course of the same operation). This must be fixed to the cartilage tissue with a watertight seal of suture and fibrin (autologous or allogenic), so as to create a chamber into which the autologous cell suspension can be injected. Indeed, if the chamber is not perfectly sealed, the cells will leak out again and the operation will have failed.
To summarise, the main disadvantages of this procedure are that the operation is difficult to perform, the technique is invasive and the implanted cells are not perfectly differentiated. Autologous and homologous osteochondral grafts involve techniques that are surgically invasive and complex and there is a risk of viral transmission with the latter.
Other attempts at reconstructing the joint cartilage consist in implanting synthetic scaffolds containing allogenic chondrocytes, and growth factors able to stimulate proliferation of the chondrocytes.
The most frequently used synthetic scaffolds are of collagen gel, polyanhydride, polyorthoester, polyglycolic acid and the copolymers thereof. The chief disadvantage of using said scaffolds is represented by an immune response directed towards the implanted material.
There are known chondrocyte cultures in gel-scaffolds constituted by agarose, hyaluronic acid, fibrin glue, collagen and alginate.
However, said cultures in gel do not provide suitable mechanical stability to remain adhered to the site and allow the reconstruction of the cartilage structure.
Moreover, chondrocyte cultures in substances such as fibrin dedifferentiate into cells that are apparently similar to fibroblasts.
Lastly, although the gels constituted by substances such as agarose induce chondrocyte redifferentiation, the use of this compound has not been approved for internal application in humans.
As previously described joint cartilage defects have also been treated with isolated chondrocyte suspensions in the absence of supporting scaffolds. It is thought, however, that chondrocytes lose their viability and/or do not remain in the defect, and that they form fibrocartilage or islets of cartilage immersed in fibrous tissue (U.S. Pat. No. 5,723,331).
To overcome this problem, the Applicant has devised injectable compositions containing chondrocytes or cells of bone marrow stroma dispersed in a matrix containing at least one hyaluronic acid derivative (PCT patent application, publication No. WO00/37124).
As is known, hyaluronic acid plays a vital role in many biological processes, such as tissue hydration, proteoglycan organisation, cell differentiation, proliferation and angiogenesis (J. Aigner et al. L. Biomed. Mater. Res. 1998, 42, 172-181).
Also known is the use of hyaluronic acid derivatives prepared as described in EP patent No. 0216453 B1 for the preparation of three-dimensional scaffolds in the form of non-woven fabrics, membranes, sponges, granules, microspheres, tubes, gauzes, for the in vitro growth of stem and mesenchymal cells (PCT patent application publication No. WO 97/18842), in the form of a nonwoven fabric associated with a perforated membrane for the growth in vitro of fibroblasts and keratinocytes (PCT patent application No. WO 96/33750 and in the form of a nonwoven fabric for the growth of chondrocytes (J. Aigner et al. L. Biomed. Mater Res. 1998, 42, 172-181).